Photoinduced Proton and Charge Transfers in a Dihydroxynaphthalene Derivative:  Chromotropic Acid

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 105; no. 46; pp. 10613 - 10620
• Main Authors: Bardez, Elisabeth, Alain, Valérie, Destandau, Émilie, Fedorov, Aleksandre, Martinho, José M. G
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22-11-2001
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp0112673

Chromotropic acid (4,5-dihydroxynaphthalene-2,7-disulfonic acid) is a chelating agent whose fluorogenic character has never been explained yet. The excited-state behavior of chromotropic acid itself has never been reported, despite its relationship with naphthols that are well-known photoacids. The present paper is accordingly devoted to a thorough investigation of chromotropic acid emission in aqueous solutions, which revealed its excited-state prototropic behavior in relation with the key role of the ground-state internal H-bond between the two −OH groups (displayed by the very different pKvalues, 5.4 and 15.6). Experiments were carried out in the whole acidity range (8 M HClO4 to 10 M NaOH) using steady-state and time-resolved fluorimetry. In concentrated HClO4, competitive quenching processes occurred, due to HClO4 itself and to water, respectively, but no photoinduced deprotonation was observed. The former quenching was confirmed by studying the emission of 2,6-naphthalenedisulfonic acid in the same media. Photoinduced deprotonation from the first −OH group was only observed in diluted acidic solutions ([HClO4] ≤ 1 M), within 500 ps, leading to an intermediate species IS*, which in turn leads to the monoprotonated form HCA3-* within ≈230 ps. The structure of HCA3-* is resonance stabilized by an internal charge transfer. The lifetime of HCA3-* is 2.1 ns. The structure of IS* was tentatively proposed to be an ion pair between H+ and the hydroxylate form of HCA3-*. The pK* value of the first −OH group (≈1.8) was deduced from the rate constants. A moderate photoacidity (ΔpK ≈ −3.6 pK units on excitation) is then displayed. Regarding the second −OH group, its photoinduced deprotonation was never observed, even in strongly basic solutions (1 M ≤ [NaOH] ≤ 10 M), which points out the outstanding stability of HCA3-*. The lifetime of the fully deprotonated form (1.76 ns) could only be measured after direct excitation in 10 M NaOH.